Unfortunately NTL::RR doesn't quite satisfy our conceptual
requirements, so there is a very thin wrapper class boost::math::ntl::RR defined in boost/math/bindings/rr.hpp
that you should use in place of NTL::RR. The
class is intended to be a drop-in replacement for the "real" NTL::RR
that adds some syntactic sugar to keep this library happy, plus some of the
standard library functions not implemented in NTL.

For those functions that are based upon the Lanczos
approximation, the bindings defines a series of approximations with
up to 61 terms and accuracy up to approximately 3e-113. This therefore sets
the upper limit for accuracy to the majority of functions defined this library
when used with NTL::RR.